Chapter 5 seems to be a message directed specifically to Joseph Smith and the latter-day Gentiles. The translator is explicitly instructed of Moroni to leave the sealed portion alone. We have no indication that Joseph was ever disobedient to this command or that he was even curious about the content of the sealed portion.
Bruce R. McConkie
"It was returned by Joseph Smith to Moroni, its divinely appointed custodian. Nor did even Joseph Smith either read or translate it. [The sealed portion of the plates.] We know of no one among mortals since Mormon and Moroni who have known its contents. It was known among the Nephites during the nearly two hundred years of their Golden Era. But for the present, the book is kept from us; only the portion upon which no seal was placed has been translated." (A New Witness for the Articles of Faith, p. 443 as taken from Latter-day Commentary on the Book of Mormon compiled by K. Douglas Bassett, p. 493)